This invention relates to injection lasers and, more particularly, to a circuit for controlling the operating current level of an injection laser.
Optical communication systems will play an important role in the development of future communication systems because of their large signal carrying capabilities. In such systems, information signals in either analog or digital form modulate the power output of a high intensity light source which is then transmitted over an optical transmission line such as an optical fiber. At the receiving end, the modulating signal is detected and the information signal is recovered. An injection laser can be used as a high intensity transmitting light source by varying the current through the laser in response to the information signal to be transmitted.
When analog frequency multiplexed signals are transmitted by an injection laser, the composite multiplexed analog signal modulates the laser current in the region in which the laser lases. The light power output is thus varied in accordance with the modulating signal. In order to minimize intermodulation distortion and cross-modulation between the different frequency multiplexed signals, the region over which the laser current is modulated must be within the most linear operating range of the laser. Distortion is minimized when the operating current level of the laser, for a zero magnitude modulating signal, is at the center of this linear operating range of the laser.
An object of the present invention is to operate an injection laser at a point of maximum linearity.